1. Field of the Invention
The present invention relates to a reforming type fuel cell system, which generates hydrogen enriched fuel gas by reforming the hydrocarbon base fuel such as methanol. The present invention especially relates to a fuel cell system which can be suitably used as a power source of an electric powered vehicle.
2. Prior Art
In recent years, an electric powered vehicles each carrying various driving motors instead of the conventional engine has been developed. As an example of such types of electric powered vehicles, a fuel cell system, as a power source for driving motor, carried vehicle have been sharply developed. A reforming type fuel cell system have been known as one of such a fuel cell system.
With an electric powered vehicle carrying the reforming type fuel cell, when a driver releases an accelerator pedal and an accelerator opening is set at 0, a demand power output to the fuel cell is decreased. In this case, a target value of the output current is made to sharply lessen based on the decreasing of the demand power output to the fuel cell. However when the output current of the fuel cell is sharply decreased to the target value of the output current, the wrong situations such as generation of the surplus hydrogen will be happen.
In order to avoid the occurrence of such a wrong situation, when the target value of the output current is sharply decreased, the output current from the fuel cell is controlled so as to decreased to the target value of the output current with some delay. As shown in FIG. 10, in the conventional fuel cell system, furthermore, the supply of the oxidant gas, such as air, to the fuel cell was decreased based on the decreasing ratio of the output current of the fuel cell.
When the quantity of the air supplied to the fuel cell is decreased at the same decreasing ratio of the output current, however, the quantity of the air which is exhausted from the fuel cell and supplied to the combustor within the evaporator will also be decreased. On the other hand, since the reforming reaction in the reformer progress slowly, the quantity of the hydrogen exhausted from the reformer is not decreased sharply but with a delay time. In just after sharply decreasing of the target value of the output current, the quantity of the air supplied to the combustor is temporary decreased without decreasing the quantity of the hydrogen supplied to the combustor. In such a case, the incomplete combustion of the fuel gas in the combustor may be caused by the shortage of air. So, when the incomplete combustion occurs, the harmful components, such as carbon monoxide, may be contained in the exhaust gas exhausted from the combustor.
Moreover, when exhausting the air supplied to the fuel cell out of the fuel cell, the produced water, which is produced in the power generation of the fuel cell and remaining in the fuel cell, is also drained. This drainage of the produced water is performed using the pressure caused by the air supplied to the fuel cell. As observed in the conventional fuel cell, however, when the flow rate of the air supplied to the fuel cell is decreased, the pressure in the fuel cell thus will be dropped and then the drainage of the produced water, which remains in the fuel cell, is disturbed. Consequently, since the remained water blocks the air supply line in the fuel cell, the cell voltage will be decreased.
An object of the present invention, is to provide the fuel cell system, which can prevents the imperfect combustion of the fuel gas in the combustor caused by shortage of the air exhausted from the fuel cell, even if the demand power output to the fuel cell is decreased. Also, there is an object to provide the fuel cell system which can prevents the decreasing of the cell voltage by enabling drainage of the produced water in the fuel cell certainly.
According to the present invention, which attains theses and other objects, there is provided a fuel cell system comprising: a fuel cell to which a fuel gas and an oxidant gas are supplied to generate electricity; a combustor to which an exhaust gas exhausted from the fuel cell is supplied to generates a heat by combustion of the exhaust gas; a reactor which forms the fuel gas by the reforming reaction of an evaporated raw fuel which is evaporated by the heat generated in the combustor; and a flow rate controller which controls the flow rate of the oxidant gas supplied to the fuel cell, said controller decreasing the flow rate of the oxidant gas supplied to the fuel cell with delay to the output current of the fuel cell, when the output of the fuel cell decrease.
With this invention, the flow rate controller, which controls the quantity of the oxidant gas, such as air, supplied to the fuel cell, is provided. The flow rate of the oxidant gas is decreased with delay to the output current of the fuel cell by the flow rate controller, when the output, for example output current, of the fuel cell decreases. The sharply drop of the pressure in the fuel cell is thus prevented even if the output current of the fuel cell is decreased. The imperfect combustion of the fuel gas caused by the shortage of the oxidant gas in the combustor is thus prevented. Furthermore, since the flow rate of the exhaust gas exhausted from the fuel cell is not decreased, the pressure in the fuel cell is maintained. The drainage of the produced water, which is produced as a result of power generation and is remaining in the fuel cell, is thus certainly achieved by the pressure in the fuel cell. Then, the decreasing of the cell voltage of the fuel cell caused by the remaining water will be prevented.
In the fuel cell system, wherein the flow rate controller preferably keeps the flow rate of the oxidant gas at constant or preferably increases the flow rate of the oxidant gas before decreasing the oxidant gas corresponding to the decreasing of the output power of the fuel cell.
With this invention, when the flow rate of the oxidant gas supplied to the fuel cell is decreased with delay on corresponding to reducing quantity of the output current of the fuel cell, the flow rate of the oxidant gas is decreased after making the flow rate of oxidant gas maintain or increase temporally. Therefore, since more oxidant gas will be supplied to the fuel cell even if the output current of the fuel cell will be decreased, more exhaust gases can be supplied to the combustor through the fuel cell. Then, the imperfect combustion of the fuel gas in the combustor is prevented certainly.
Furthermore, since the quantity of the exhaust gas exhausted from the fuel cell will be safe, the produced water generated by the power generation and remaining in the fuel cell is certainly drained.
In the fuel cell system, the flow rate controller controls the flow rate of the oxidant gas based on the oxidant gas flow rate signal computed from the target output current of the fuel cell and target value of the fuel gas of the exhaust gas exhausted from the reactor.
With this invention, in the case that the flow rate of the oxidant gas is controlled using the flow rate controller, the oxidant gas flow rate signal computed by the target value of output current and the target output value of exhausted fuel is used. The flow rate of the oxidant gas is thus determined without using the another equipment.